Sanction, no.
Classless? In Djokovic's case?
Well...
When Andrew Ilie did it, it was funny.
When Goran did it, it was cool.
If Federer ever does it (and he will not), it will be considered masterful.
IF Rafa ever does it, it will be Guido-ish, but strrangely humble.
If Roddick ever did it, it would be arrogant and ugly. Same goes for Becker.
If Todd Martin did it, he'd totally get away with it, in a Fred MacMurray way.
If Serena did it, it'd be gangsta....or ghetto or whatever.
If Guga Kuerten or Pat Rafter ever did it, it would be considered pure joy pouring out of a champion's soul.
So, it's really not the shirtlessness. It's the ***** inside the shirt.